Heterotopic ossification (HO) involves unwanted bone growth that may be characterized by inappropriate differentiation of cells into bone-forming cells. This condition leads to bone formation, usually near joints, where the bone formation often limits the mobility of the joint. HO may follow neurological injury and direct injury to soft tissue such as muscles or connective tissue around the joint in which HO later develops.
There are three recognized etiologies of HO: traumatic, neurogenic, and genetic. Traumatic HO typically follows fractures, dislocations, operative procedures, and severe burns. Most commonly, HO is seen around the hip after fracture and open reduction-internal fixation (ORIF) procedures or total hip arthroplasties (THA). As well, HO is often associated with pathologies such as traumatic brain injury (TBI), spinal cord injury (SCI), infections of the central nervous system (CNS), tumors, strokes, tetanus, polio, tabes dorsalis, multiple sclerosis, and selective posterior rhizotomy. The presence of idiopathic muscle spasticity is also associated with the development of HO.